196 research outputs found
A 2D pixelated stilbene scintillator detector array for simultaneous radiography with fast neutrons and gammas
For radiography applications using fast neutrons simultaneously with gammas
we have developed a detector with 16 stilbene crystals in a 44 2D array
with a 5~mm pitch and a depth of 25~mm. The crystal array is read out by
Silicon photomultipliers and custom signal processing electronics. The detector
prototype was tested using a custom D-D fast neutron generator at the Paul
Scherrer Institute. By applying a pulse shape discrimination algorithm the
detector is able to detect and distinguish fast neutrons and gammas
simultaneously. Various attenuating samples placed between the source and
detector with different materials and thicknesses were tested and the measured
macroscopic fast neutron cross sections were compared to what was expected.
Deviations were studied with the help of detailed Geant4 simulations. The
detection efficiency for D-D fast neutrons was measured to be around 10\%.Comment: Prepared for submission to JINST. Changes compared to previous
version: article was edited due to JINST referee feedback (minor revision
requested). Figure 3 was updated. Formulations were improved, some
explanations were given in more detail. Reference list was updated (reference
15 was exchanged
A novel copper precursor for electron beam induced deposition
A fluorine free copper precursor, Cu(tbaoac)2 with the chemical sum formula CuC16O6H26 is introduced for focused electron beam induced deposition (FEBID). FEBID with 15 keV and 7 nA results in deposits with an atomic composition of Cu:O:C of approximately 1:1:2. Transmission electron microscopy proved that pure copper nanocrystals with sizes of up to around 15 nm were dispersed inside the carbonaceous matrix. Raman investigations revealed a high degree of amorphization of the carbonaceous matrix and showed hints for partial copper oxidation taking place selectively on the surfaces of the deposits. Optical transmission/reflection measurements of deposited pads showed a dielectric behavior of the material in the optical spectral range. The general behavior of the permittivity could be described by applying the MaxwellâGarnett mixing model to amorphous carbon and copper. The dielectric function measured from deposited pads was used to simulate the optical response of tip arrays fabricated out of the same precursor and showed good agreement with measurements. This paves the way for future plasmonic applications with copper-FEBID
Properties of the ultraviolet flux of type Ia supernovae: an analysis with synthetic spectra of SN 2001ep and SN 2001eh
The spectral properties of type Ia supernovae in the ultraviolet (UV) are
investigated using the early-time spectra of SN 2001ep and SN 2001eh obtained
using the Hubble Space Telescope (HST). A series of spectral models is computed
with a Monte Carlo spectral synthesis code, and the dependence of the UV flux
on the elemental abundances and the density gradient in the outer layers of the
ejecta is tested. A large fraction of the UV flux is formed by reverse
fluorescence scattering of photons from red to blue wavelengths. This process,
combined with ionization shifts due to enhanced line blocking, can lead to a
stronger UV flux as the iron-group abundance in the outer layers is increased,
contrary to previous claims.Comment: 14 pages, 13 figures. Replaced with revised version accepted for
publication in MNRA
Neutral and charged excitons interplay in non-uniformly strain-engineered WS2
We investigate the response of excitons in two-dimensional semiconductors to nonuniformity of mechanical strain. In our approach to non-uniform strain-engineering, a WS2 monolayer is suspended over a triangular hole. Large (>2%), strongly non-uniform (>0.28% ”mâ1), and in-situ tunable strain is induced in WS2 by pressurizing it with inert gas. We observe a pronounced shift of the spectral weight from neutral to charged excitons at the center of the membrane, in addition to well-known strain-dependent bandgap modification. We show that the former phenomenon is a signature of a new effect unique for non-uniform strain: funneling of free carriers towards the region of high strain followed by neutral to charged exciton conversion. Our result establishes non-uniform strain engineering as a novel and useful experimental 'knob' for tuning optoelectronic properties of 2D semiconductors
Towards tunable graphene phononic crystals
Phononic crystals (PnCs) are artificially patterned media exhibiting bands of allowed and forbidden zones for phononsâin analogy to the electronic band structure of crystalline solids arising from the periodic arrangement of atoms. Many emerging applications of PnCs from solid-state simulators to quantum memories could benefit from the on-demand tunability of the phononic band structure. Here, we demonstrate the fabrication of suspended graphene PnCs in which the phononic band structure is controlled by mechanical tension applied electrostatically. We show signatures of a mechanically tunable phononic band gap. The experimental data supported by simulation suggests a phononic band gap at 28â33 MHz in equilibrium, which upshifts by 9 MHz under a mechanical tension of 3.1 N mâ1. This is an essential step towards tunable phononics paving the way for more experiments on phononic systems based on 2D materials
On the evolution of rapidly rotating massive white dwarfs towards supernovae or collapses
A recent study by Yoon & Langer (2004a) indicated that the inner cores of
rapidly accreting (Mdot > 10^{-7} M_sun/yr) CO white dwarfs may rotate
differentially, with a shear rate near the threshold value for the onset of the
dynamical shear instability. Such differentially rotating white dwarfs obtain
critical masses for thermonuclear explosion or electron-capture induced
collapse which significantly exceed the canonical Chandrasekhar limit. Here, we
construct two-dimensional differentially rotating white dwarf models with
rotation laws resembling those of the one-dimensional models of Yoon & Langer
(2004a). We derive analytic relations between the white dwarf mass, its angular
momentum, and its rotational-, gravitational- and binding energy. We show that
these relations are applicable for a wide range of angular velocity profiles,
including solid body rotation. We demonstrate that pre-explosion and
pre-collapse conditions of both, rigidly and differentially rotating white
dwarfs are well established by the present work, which may facilitate future
multi-dimensional simulations of Type Ia supernova explosions and studies of
the formation of millisecond pulsars and gamma-ray bursts from collapsing white
dwarfs.Our results lead us to suggest various possible evolutionary scenarios
for progenitors of Type Ia supernovae, leading to a new paradigm of a variable
mass of exploding white dwarfs, at values well above the classical
Chandrasekhar mass. Based on our 2D-models, we argue for the supernova peak
brightness being proportional to the white dwarf mass, which could explain
various aspects of the diversity of Type Ia supernovae, such as their variation
in brightness, the dependence of their mean luminosity on the host galaxy type,
and the weak correlation between ejecta velocity and peak brightness.Comment: Based on 2-D white dwarf models: 19 pages, 13 figures, A&A, accepte
The first binary star evolution model producing a Chandrasekhar mass white dwarf
Today, Type Ia supernovae are essential tools for cosmology, and recognized
as major contributors to the chemical evolution of galaxies. The construction
of detailed supernova progenitor models, however, was so far prevented by
various physical and numerical difficulties in simulating binary systems with
an accreting white dwarf component, e.g., unstable helium shell burning which
may cause significant expansion and mass loss. Here, we present the first
binary evolution calculation which models both stellar components and the
binary interaction simultaneously, and where the white dwarf mass grows up to
the Chandrasekhar limit by mass accretion. Our model starts with a 1.6 Msun
helium star and a 1.0 Msun CO white dwarf in a 0.124 day orbit. Thermally
unstable mass transfer starts when the CO core of the helium star reaches 0.53
Msun, with mass transfer rates of 1...8 times 10^{-6} Msun/yr. The white dwarf
burns the accreted helium steadily until the white dwarf mass has reached ~ 1.3
Msun and weak thermal pulses follow until carbon ignites in the center when the
white dwarf reaches 1.37 Msun. Although the supernova production rate through
this channel is not well known, and this channel can not be the only one as its
progenitor life time is rather short (~ 10^7 - 10^8 yr), our results indicate
that helium star plus white dwarf systems form a reliable route for producing
Type Ia supernovae.Comment: 4 pages, 5 figure
Interpreting the near-infrared spectra of the 'golden standard' Type Ia supernova 2005cf
We present nine near-infrared (NIR) spectra of supernova (SN) 2005cf at
epochs from -10 d to +42 d with respect to B-band maximum, complementing the
existing excellent data sets available for this prototypical Type Ia SN at
other wavelengths. The spectra show a time evolution and spectral features
characteristic of normal Type Ia SNe, as illustrated by a comparison with SNe
1999ee, 2002bo and 2003du. The broad-band spectral energy distribution (SED) of
SN 2005cf is studied in combined ultraviolet (UV), optical and NIR spectra at
five epochs between ~ 8 d before and ~ 10 d after maximum light. We also
present synthetic spectra of the hydrodynamic explosion model W7, which
reproduce the key properties of SN 2005cf not only at UV-optical as previously
reported, but also at NIR wavelengths. From the radiative-transfer calculations
we infer that fluorescence is the driving mechanism that shapes the SED of SNe
Ia. In particular, the NIR part of the spectrum is almost devoid of absorption
features, and instead dominated by fluorescent emission of both iron-group
material and intermediate-mass elements at pre-maximum epochs, and pure
iron-group material after maximum light. A single P-Cygni feature of Mg II at
early epochs and a series of relatively unblended Co II lines at late phases
allow us to constrain the regions of the ejecta in which the respective
elements are abundant.Comment: 11 pages, 6 figures, accepted for publication in MNRA
Circumstellar interaction of the type Ia supernova 2002ic
We propose a model to account for the bolometric light curve, quasi-continuum
and the Ca II emission features of the peculiar type Ia supernova (SN) 2002ic,
which exploded in a dense circumstellar envelope. The model suggests that the
SN Ia had the maximum possible kinetic energy and that the ejecta expand in an
approximately spherically symmetric (possibly clumpy) circumstellar
environment. The Ca II and quasi-continuum are emitted by shocked SN ejecta
that underwent deformation and fragmentation in the intershock layer. Modeling
of the Ca II triplet implies that the contribution of the O I 8446 A line is
about 25% of the 8500 A feature on day 234, which permits us to recover the
flux in the Ca II 8579 A triplet from the flux of 8500 A blend reported by Deng
et al. (2004). We use the Ca II doublet and triplet fluxes on day 234 to derive
the electron temperature (~4400 K) in the Ca II line-emitting zone and the
ratio of the total area of dense fragments to the area of the shell, S/S_0 ~
100. We argue that Ca II bands and quasi-continuum originate from different
zones of the shocked ejecta that reflect the abundance stratification of the
supernova.Comment: 12 pages, MNRAS, in pres
Detection of CO and Dust Emission in Near-Infrared Spectra of SN 1998S
Near-infrared spectra (0.95 -- 2.4 micron) of the peculiar Type IIn supernova
1998S in NGC 3877 from 95 to 355 days after maximum light are presented. K-band
data taken at days 95 and 225 show the presence of the first overtone of CO
emission near 2.3 micron, which is gone by day 355. An apparent extended blue
wing on the CO profile in the day 95 spectrum could indicate a large CO
expansion velocity (~2000 -- 3000 km/s). This is the third detection of
infrared CO emission in nearly as many Type II supernovae studied, implying
that molecule formation may be fairly common in Type II events, and that the
early formation of molecules in SN 1987A may be typical rather than
exceptional. Multi-peak hydrogen and helium lines suggest that SN 1998S is
interacting with a circumstellar disk, and the fading of the red side of this
profile with time is suggestive of dust formation in the ejecta, perhaps
induced by CO cooling. Continuum emission that rises towards longer wavelengths
(J -> K) is seen after day 225 with an estimated near-infrared luminosity >~
10^40 erg/s. This may be related to the near-infrared excesses seen in a number
of other supernovae. If this continuum is due to free-free emission, it
requires an exceptionally shallow density profile. On the other hand, the shape
of the continuum is well fit by a 1200 +- 150 K blackbody spectrum possibly due
to thermal emission from dust. Interestingly, we observe a similar 1200 K
blackbody-like, near-infrared continuum in SN 1997ab, another Type IIn
supernova at an even later post-maximum epoch (day 1064+). A number of dust
emission scenarios are discussed, and we conclude that the NIR dust continuum
is likely powered by the interaction of SN 1998S with the circumstellar medium.Comment: 38 Pages, 12 Figures, Submitted to The Astronomical Journa
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